The invention relates to a position measuring instrument and, more particularly, to a position measuring instrument having two sequences of different reference marks extending in parallel to an incremental division track.
Measuring instruments of this kind are used particularly in machine tools to measure the relative location of a tool. They are also used in coordinate measuring machines to ascertain the location and size of test specimens.
The electrical pulses generated at the reference marks can be used for reproducing reference positions in the counter, for approaching a certain position at the beginning of a measurement or after interruptions, for monitoring and correcting the counter state, and for acting upon a control unit on the output side.
For ascertaining the reference positions in incremental measuring instruments, two fundamental principles are known. By the first principle, a row of identical reference marks is applied at fixed intervals parallel to the incremental division. A switch means is assigned to each reference mark that is intended to become operative in the measuring operation. The electrical outputs of the switch means and of the scanner element for the reference marks are connected to an evaluation unit, which outputs a control pulse to the counter only whenever an electrical signal is present simultaneously both at the scanner element of the reference mark and at the switch means. The result of the control pulse is that a predetermined counter value is set in the counter. A measuring instrument of this kind is disclosed in U.S. Pat. Nos. 4,101,764 and 4,363,964 which are hereby specifically incorporated herein.
By the second principle, at least one reference mark track with identical reference marks is disposed, again parallel to the incremental division. Each reference mark is spaced apart from another reference mark by a special interval that characterizes the reference mark. These different intervals between reference marks are determined by scanning the incremental division by means of counting the measuring steps (increments). One such measuring instrument is known from U.S. Pat. No. 3,982,106 which is hereby specifically incorporated herein. In U.S. Pat. No. 4,793,067 which is hereby specifically incorporated herein two successive intervals between reference marks are used to determine the absolute value of a reference mark.
In German Patent Disclosure DE 42 43 778 A1, a position measuring instrument is again described in which an incremental division is provided to form counting signals, and a further track is provided to determine the absolute position. This further track, in an exemplary embodiment, comprises a sequence of identical reference marks at different intervals, so that the various reference mark positions can be distinguished from one another by the characteristic spacing between two successive reference marks. In another exemplary embodiment, the further track comprises markings, which form a sequential code for generating position-determining code words.
The attempt has already been made to create measuring instruments in which both principles are combined in a single position measuring instrument. One of these measuring instruments is described in U.S. Pat. No. 4,999,623 which is hereby specifically incorporated herein. Parallel to the incremental track, there is a reference mark track in which only identical reference marks are disposed, at successive different intervals. By counting out the increments from one reference mark to the next, the absolute position of these marks is determined. A further track has a row of markings that are disposed at equal intervals. To make a selection of reference marks, these markings are assigned to certain reference marks. From these markings in turn, one can be selected, as a result of which only the reference mark assigned to it is made to become operative. A mode selector toggle switch is also provided, with which the evaluation of all the reference marks by the principle of interval coding, or only the evaluation of the marked reference marks, is enabled.
The disadvantage of this position measuring instrument is that to select reference marks spaced apart by equal intervals, one additional track is needed, as well as a third track for further selection of one of the markings. Moreover, the markings must be assigned precisely to the reference marks, which requires expensive adjustment work. Another disadvantage is that when the lengths to be measured are great, successive reference marks must have a large interval between them, if different intervals over the entire measurement length are to be attained. The shortest possible interval between markings in the second track corresponds, however, to the greatest interval between reference marks, so that the reference marks selected by means of the markings have an overly long interval for actual practice if the second measurement principle is to be employed.
To overcome this last disadvantage, German Patent Disclosure DE 42 44 178 A1 provides at least two tracks that have reference marks. This has the disadvantage that a relatively large amount of space must be available next to the incremental division, and that the scanning unit is very wide.
An object of the present invention is to reduce production and storage costs and to assure broad utility by making the position measuring instrument uniform, and at the same time to attain a low structural height of the position measuring instrument.
Advantages of the invention are that the position measuring instrument can be adapted to the requirements of the user and nevertheless have a low structural height.